Mectin insecticides are used in a wide variety of crop protection applications including field crops, fruit trees, cotton and ornamentals, with the most common use being in citrus crops and pome fruits. Mectins are used typically to target pests such as mites (e.g., red mites), some members of the Lepidoptera family and some dipterous leafminers. Another common target for control with mectins is nematodes, generally in a seed treatment and in combination with another insecticide and/or fungicide. Exemplary mectins include abamectin, emamectin, ivermectin, doramectin, eprinomectin and selamectin, though several are primarily used in animal health. Mectins have a common chemical structural feature; they are macrocyclic lactones, generally including a base 16-membered ring. This structure is shared with two other groups of insecticides, milbemycins, described below, and spinosyns.
Closely related to mectin insecticides are milbemycin insecticides and spinosyn insecticides. Milbemycins are commonly used to control nematodes in tea crops and in silviculture, amongst other crop protection applications. Exemplary milbemycins include but are not limited to milbemectin, lepimectin, moxidectin, milbemectin oxime. Also related to mectins are spinosyn insecticides (e.g., spinosad and spinetoram) which are related to mectin and milbemycins by chemical structure and origin, but differ in their mode of action. Mectins and milbemycins act as chlorine channel activators but spinosyns are nicotinic acetylcholine receptor allosteric activators.
Mectins milbemycins are fermentation products of soil microorganisms of the Streptomyces genus. Eight different mectins can be isolated, and the eight different compounds form four pairs of homologs, though one member of the pair generally found in much higher abundance that the minor member of the pair. Exemplary ratios are generally greater than 4 to 1. Likewise, milbemycins are found in homolog pairs. Abamectin was the first candidate found to have insecticide properties and was the first introduced to the market. In addition to being an insecticide, abamectin has shown acaricide and nemacide properties when mites and nematodes are targeted. Emamectin, a derivative of abamectin was subsequently developed and marketed for control of Lepidoptera. Other mectins include ivermectin, eprinomectin, doramectin and selamectin, which are typically used in animal husbandry as antiparasitic drugs.
Mectins and milbemycins are very potent against mites, insects and nematodes, with lethal concentration (LC90) values in the range of 0.1 to 0.01 ppm. The efficacy of the insecticides within each class and among these two classes can vary. Despite the lethality of these compounds, they do pose some drawbacks for users including rapid degradation, photolysis and low soil motility and thus rapid soil degradation by soil microorganisms. Some compounds are taken up by the treated plant, particularly within leaves, and demonstrate some residual activity against mites, however, macrocyclic lactones are not phloem and xylem mobile, and therefore do not show a true systemic effect.